The primary medical instrument used to monitor the electrical activity of a patient's brain is the electroencephalograph (EEG). EEGs monitor brain activity by measuring the very small voltage fluctuations that are generated in the brain, which are detected by electrodes attached to a patient's scalp. To aid in studying these analog signals, a record of the voltage fluctuations (called an electroencephalogram) is often made over time. Traditionally, electroencephalograms are made using a mechanical EEG recorder that employs pens to record the analog voltage fluctuations on a strip of paper. As a continuous chart of paper is moved beneath an array of 10 to 24 galvanometer-driven ink pens, the pens trace out the brain wave activity as a series of wavy or jagged lines.
As a by-product of the recording process, pen-on-paper EEG recorders produce a significant amount of auditory noise. Most of the noise is produced by the pens of the recorder, which generate distinctive sounds as they sweep back and forth across the paper and collide with each other or with mechanical stops. A lesser amount of noise is produced by the paper feed mechanism, which emits a continuous "hum." The total noise generated by a pen-on-paper EEG recorder depends upon the electrical activity of the brain of the patient plus artifact from muscle or electrode movement. When a patient moves or has heightened levels of brain activity, the sound generated by the pens' movement increases. When a patient is quiet and exhibits very little EEG activity, the continuous sound of the paper feed mechanism generates most of the noise.
Experienced medical technicians use the sounds produced by pen-on-paper EEG recorders to their advantage. Because the sound produced by such recorders is directly related to the incoming signals, technicians can correlate distinctive pen noises with changing brain wave patterns. For example, certain high amplitude brain activity such as epileptiform discharges (without seizure activity) will cause a pen-on-paper EEG recorder to produce a characteristic sound signature. As a result, medical technicians monitoring a patient can therefore rely on recorder sounds for information regarding the patient's mental state rather than continuously observing the output traces produced by the recorder. This allows the technicians to visually monitor the patient while aurally monitoring electrical brain activity.
In recent years, analog EEGs and their associated mechanical pen-on-paper recorders have begun to be replaced by EEGs that process electroencephalogram signals digitally. Digital EEGs convert the analog brain waves into digital signals that are stored in digital form in computer memory, or on computer disk or tape. The digital data is displayed as brain wave traces on a video screen. Such systems are nearly silent, thus depriving trained medical technicians of a valuable source of information. Even when the digitally stored information is printed out using a dot-matrix printer, a thermal printer or a laser printer, sounds comparable to those created by pen-on-paper EEG recorders are not created. Further, because these sounds are absent, medical technicians are forced to watch the screen of the display rather than watch the patient. Thus, medical technicians may miss information they would have obtained if they had been observing a patient during a critical interval.
The present invention is directed to overcoming the foregoing disadvantage by adding the characteristic audible sounds of a pen-on-paper EEG recorder to the visual output of digital EEGs. As a result, digital EEG systems incorporating the invention will provide the same (or better) information than that provided by prior analog EEG systems. That is, the sounds produced may take one, or both, of two forms. They may mimic the sounds of moving pens on paper, exactly reproducing the sound that a pen-on-paper EEG recorder attached to an analog EEG would make if receiving the same brain wave signals. Or the sounds may be new and perhaps more distinctive to correlate with changing brain wave patterns.